Tumor Necrosis Factor, a pleiotropic cytokine released by activated T cells and macrophages, is expressed as a mature 17 kDa protein that is active as a trimer (Smith, R. A. and Baglioni, C., J. Biol. Chem., 262:6951 (1986). Trimeric cytokines such as Tumor Necrosis Factor (TNFα) and the closely related protein lymphotoxin (TNFβ), exert their biological activity by aggregating their cell surface receptors. The TNF trimer binds the receptors on the cell surface causing localized crosslinking of TNF receptors into clusters necessary for signal transduction.
The action of TNFα and TNFβ are mediated through two cell surface receptors, the 55 kDa (p55 TNF-R) and the 75 kDa (p75 TNF-R) receptors. Truncated forms of these receptors, comprising the extracellular domains (ECD) of the receptors, have been detected in the urine and serum as 30 kDa and 40 kDa TNF inhibitory binding proteins (Engelmann, H., et al., J. Biol. Chem., 265:1531 (1990)).
TNF is a key mediator in a number of autoimmune and inflammatory diseases such as rheumatoid arthritis, septic shock, cerebral malaria and multiple sclerosis (reviewed in Tracy, K. J. and Cerami, A., Ann. Rev. Cell. Biol., 9:317 (1993)). Antagonistic TNF treatment with anti-TNF antibodies and dimeric TNF-receptor-IgG fusion chimeras have shown promising therapeutic results for a variety of diseases in animal models (Lesslauer, W., et al., Eur. J. Immunol., 21:2883 (1991); Kolls, J., et al., Proc. Natl. Acad. Sci. USA, 91:215 (1994); Baker, D., et al., Eur. J. Immunol., 24:2040 (1994); Williams, R. O., et al., Proc. Natl. Acad. Sci. USA, 89:9784 (1993)) and human clinical trials (Elliot, M., et al., Arthritis and Rheum., 36:1681 (1993)).
For example, it has been shown that the IgG-Hu p75 TNF-R ECD dimers have a 100–4000 fold higher affinity for TNF over the monomeric counterparts (Lesslauer, W., et al., Eur. J. Immunol., 21:2883 (1991); Kolls, J., et al., Proc. Natl. Acad. Sci. USA, 91:215 (1994); Butler, D., et al., Cytokine, 6:616 (1994)). However, these molecules are large in size, immunogenic and include the Fc portion of the IgG which may interfere with clearance by binding to Fc receptors.
Thus, a need exists for improved TNF inhibitors which are less immunogenic and allow for faster clearance and greater tissue penetration when administered to a host.